1. Field of the Invention
The invention relates to a method of determining the efficiency of asynchronous motors.
The invention also relates to an apparatus for carrying out the method.
2. Description of the Related Art
The determination of the efficiency of three-phase asynchronous machines can be carried out by directly measuring the input power and the output power or by determining the output power by means of the loss summation method. These methods are defined in DIN 57 530 Part 2/VDE 0530 Part 2/11.82.
When the input power and the output power are measured directly, the input power is determined by utilizing analog or digital measuring devices for determining the actual power. The output power is computed by measuring the torque and the rate of rotation and the subsequent multiplication in accordance with the formula P=2.times..pi..times.M.times.n. The efficiency .eta. then results as the quotient of output power and input power.
When the efficiency is determined indirectly, initially the no-load characteristic of an asynchronous motor is determined in the no-load test. In that case, the asynchronous motor which runs without load uses from the network only the power necessary for covering the internal losses. These losses are divided into no-load copper losses, the iron losses as well as the friction losses. In accordance with generally known methods, the individual loss portions are then separated from the no-load characteristic. For this purpose, the input of the characteristic P=f(U), I=f(U) takes place; in other words, the asynchronous motor running without loads must be operated at different voltages. Subsequently, the efficiency of the asynchronous motor can be determined with sufficient accuracy for the respective load point during load operation and the rated efficiency can be determined for the rated load point.
Accordingly, the rated efficiency .eta. results from the division of the shaft power P.sub.2 and the power P.sub.1 consumed by the motor. The shaft power P.sub.2 is computed from the relationship P.sub.2 =P.sub.1 -V.sub.Fe -V.sub.Cu -V.sub.Zus -V.sub.Cu2 -V.sub.Rbg. In this relationship, V.sub.Cu2 are the rotor copper losses and V.sub.Rbg are the friction losses which can be determined from the no-load characteristic. The rotor copper losses V.sub.Cu2 can be determined from the relationship s.times.P.sub.d in which P.sub.d means the air gap power transferred from the motor stator into the rotor and s means the slip in the rated point of the motor. V.sub.Fe are the iron losses from the no-load test, while V.sub.Zus are the additional losses which, depending on the motor, are in an order of magnitude of between 0.5% and 2% of the power consumed by the motor. The copper losses V.sub.Cu are computed from the relationship 1.5.times.I.sup.2 .times.R, wherein R means the terminal resistance of the motor and I means the current consumed by the motor in the rated point.
When utilizing the known methods, it is possible to determine exactly the rated efficiency of an asynchronous motor. However, this determination requires a relatively long time and the use of appropriate measuring devices; in this connection, it should be mentioned that it is sufficient in practical operation of an asynchronous motor to know whether the efficiency is within a certain range. In this manner, a judgment test concerning the economic efficiency of the operated units is obtained which is sufficient in practice.